1. Field of the Invention
This invention relates to electronic pressure transmitters having semiconductor strain-gauge type pressure sensors, and more particularly to a circuit which operates to compensate for pressure measurement errors created by temperature.
2. Description of the Prior Art
A semiconductor pressure transmitter typically includes a pressure sensor comprising piezoresistive elements diffused into a silicon diaphragm. The operation of the transmitter is such that a pressure differential develops a strain in the silicon diaphragm which strain produces a change in the resistance of the piezoresistive elements. If a pair of piezoresistive elements are diffused orthogonally with respect to one another in the silicon diaphragm, the pressure differential when applied across the diaphragm causes the resistance of one piezoresistive element to increase an amount and the other to decrease by an equal amount. In other words, a transversely oriented piezoresistive element when subjected to a strain changes its resistance by an amount equal in magnitude but opposite in sign to the change in resistance resulting from the application of the same strain to a longitudinally oriented element.
With reference to the drawings appended in this application, Wheatstone bridge 10 shown in FIG. 1 is a pressure sensor circuit typically used in semiconductor pressure transmitters of the prior art. Piezoresistive elements R.sub.1 through R.sub.4 are diffused into a silicon diaphragm (not shown). If bridge 10 is connected to a constant current source (not shown) which produces current I and is then subjected to a strain, the resistances of elements R.sub.1 through R.sub.4 will individually change so that a voltage V.sub.O measured at terminal nodes 20 and 22 will change. Bridge voltage V.sub.B measured at nodes 12 and 14 will remain essentially unchanged because elements R.sub.1 through R.sub.4 are orthogonally arranged so that the resistance change of one resistor is compensated by the resistance change of another resistor. Since voltage V.sub.O varies in direct relation to the strain being applied to bridge 10, it is useful as a measure of the magnitude of the pressure differential being applied to the diaphragm. However, as is well known, the resistance of elements R.sub.1 through R.sub.4 are temperature dependent. Line 30 in FIG. 2A is a typical plot of the changes in bridge voltage V.sub.B as a function of temperature and with constant bridge current. For a specific pressure transmitter, bridge voltage V.sub.B may vary, for example, as much as 30 percent of the entire voltage range for a 100.degree. C. change in temperature.
Two measures of temperature performances of pressure transmitters are zero error and span error. As further explanation, zero error refers to the percent change in voltage V.sub.O with respect to the voltage produced at a prescribed temperature T.sub.Ref and under the conditions of zero pressure differential acting on the diaphragm. With reference to line 32 shown in FIG. 2B, percent zero error is the voltage change expressed as a percent of the voltage occurring at temperature T.sub.Ref. Span is the difference between the maximum and minimum values of a prescribed range of pressures over which the pressure transmitter is designed to measure. With reference to line 34 shown in FIG. 2C, percent span error is the difference, expressed as a percent of a reference (or prescribed) span, between the actual span and the reference span. It should be noted that the direction of change of the zero and span errors are not related to one another. In other words, zero error may vary negatively or positively as depicted by lines 32 and 33 respectively and span error may vary positively or negatively as depicted by lines 34 and 35 respectively.
In FIGS. 3 and 4A-B there are shown respectively a prior typical art pressure sensor circuit which includes resistors to compensate for zero errors as well as span errors and graphs depicting the corresponding percent zero and percent span errors. To compensate for zero error, such as depicted by line 32 shown in FIG. 2B, series resistor R.sub.S and parallel resistor R.sub.P are included in the pressure sensor circuit so that percent zero error line 38 (shown in FIG. 4A) crosses the zero percent line at two specified temperatures T.sub.A and T.sub.B, that may be the end points of the temperature range in which the pressure sensor circuit is designed to operate. Resistors R.sub.S and R.sub.P are temperature stable, that is, their resistances do not vary over the temperature range between T.sub.A and T.sub.B.
The specific resistance values for R.sub.S and R.sub.P may be determined by first measuring empirically the actual zero error at the two prescribed temperatures T.sub.A and T.sub.B for the pressure sensor circuit comprising only R.sub.1 through R.sub.4 in a bridge arrangement and second using circuit analysis techniques, which are well known in the art, for the series and parallel connections of resistors R.sub.S and R.sub.P to determine the required resistance values for eliminating the two empirically measured errors.
It should be noted that resistors R.sub.1 through R.sub.4 of sensor 40 are not orthogonally arranged with one another but are positioned in a bridge configuration in the diaphragm so that a pressure differential when applied to the diaphragm places two of the four resistors in compression and the remaining two resistors in tension. Accordingly, two piezoresistive elements increase in resistance and two other piezoresistive elements decrease in resistance.
In addition, it should be recalled that in FIG. 2B the percent zero error line may vary positively or negatively. As a result, resistor R.sub.S may be located between nodes 24 and 25 and a short connected between nodes 23 and 24 (or vice versa as required) and resistor R.sub.P may be connected between nodes 28 and 29 and nodes 26 and 27 left open therebetween (or vice versa as required) to compensate for the percent zero error.
In order to compensate for span errors, such as shown by line 34 in FIG. 2C, temperature-stable resistor R.sub.SPAN is connected across nodes 12 and 14 where bridge voltage V.sub.B appears. Using a two-step process similar to that described above for determining the values of resistors R.sub.S and R.sub.P, the value of R.sub.SPAN may be found such that percent span error line 39 shown in FIG. 4B crosses the two prescribed temperature points T.sub.A and T.sub.B with the same span error percentage.
It will be recalled that two temperature-stable resistors were used to compensate for zero error. Although only one resistor is required to equalize the zero errors at temperatures T.sub.A and T.sub.B, a second resistor is used because of the additional requirement of offsetting any voltage produced at those temperatures when there is no pressure differential. This additional requirement is not needed for span error correction because the actual span does not have to equal the reference span. In other words, if the actual span is an acceptable range, then it is only important that the span does not change at the two prescribed temperatures so that span errors occurring therebetween are substantially reduced.
It should be understood from the above that the passive resistance circuits for correcting zero and span errors are characterized by continuous and smooth transfer functions and do not fully compensate for the errors occurring at temperatures intermediate T.sub.A and T.sub.B because such errors are non-linearly dependent upon temperature. In process control applications, there is a need to reduce further the errors produced by the above described pressure sensors so that pressure measurements are made with increased accuracy.
U.S. Pat. No. 4,202,218, issued to Romo on May 13, 1980, discloses another means for temperature compensation wherein active electronic components are included with the above described passive resistive circuits. The output of the pressure sensor circuit is fed back to change the output of a constant current power supply in an effort to compensate for the changes in resistance caused by temperature so that zero and span errors are thereby reduced. However, the transfer functions describing the characteristic performances of the components used in the temperature compensation circuits are all continuous and linear functions over the prescribed temperature range. Accordingly, the reductions in the percent of zero and span errors are necessarily limited because of the non-linear relationship of zero and span errors with temperature.
Still another means of compensating for zero and span temperature errors is disclosed in the IEEE Transactions on Electron Devices, Volume ED-16, No. 10, dated October 1969, in the article on pages 870-876 entitled "Solid State Digital Pressure Transducer." That article teaches the use of separate pressure and temperature sensors which produce information combined in accordance with a complex computer algorithm so that the existing pressure can be calculated with great accuracy. However, such means is necessarily costly and use of a computer program and associated hardware introduces complexities which may be deleterious to the reliability of the pressure transmitter.
Therefore, there is a need for improvement in the means for providing temperature compensation for zero and span errors of pressure transmitters.